French magistrates have placed under investigation a woman and her son suspected of stealing hundreds of thousands of euros worth of jewellery in robberies across Europe, a judicial source said on Thursday.

French police started a massive hunt for the couple after a woman and a man posing as wealthy Middle Eastern buyers stole a 5.5 carat diamond ring worth 635,000 euros ($799,100) at jewellers Cartier in Paris in November.

The 55-year-old woman, wearing a headscarf, asked to see several rings and replaced one of them with a copy.

The pair were arrested in northern France on Friday and have admitted to some crimes, the source said. Being placed under investigation is a formal step ahead of possible charges.

The judicial source said several jewels had been recovered from the couple, but not the Cartier ring. The source added that the pair had admitted to thefts in Germany and Spain.

Martin Winckel of the International Jeweller Security network in Germany said he had traced a string of similar thefts, starting with the robbery of a 210,000-euro ring from Tiffany's in Frankfurt.

"She is the best," he said, referring to a woman seen on internal store cameras who had managed to switch rings.

"Stealing 5.5 carat rings during a sales conversation means she's extremely experienced and professional," he said.

Jewellery thieves are increasingly taking advantage of a borderless Europe to expand their hunting grounds, he said.